Efforts to improve the networking of digital computers and the transmission of digital data have been the object of significant research and development in the past. Networking allows computers to share resources, access huge stores of information, communicate via e-mail, share data, and transfer files. Networking technology and digital data transmission have been subject to a number of bandwidth and speed limitations.
In the past, networking technology has suffered from limitations on data transmission rates which limit the bandwidth of the system. For example, local area networks (LANs) may be connected with cables that have finite limitations on the amount of data they can pass, and the speed at which it ca n be done. LAN's may be connected to extended wide area networks (WAN's) over transmission lines that have bandwidth limitations. When modems are required for communication over conventional telephone lines, severe limitations may be imposed upon data transmission rates.
In order for a network to accommodate a number of users efficiently, routing and flow control procedures have to be established. There are many rules that must be followed, and these rules are typically referred to as protocols. Packet-switched networks subdivide digital data messages into packets. The digital data is then transmitted packet by packet. Each packet must contain not only the information bits comprising the digital data that is to be transmitted, but also information bits which are overhead required by the protocol in use, such as information bits which identify the destination of the packet, the source of the packet, and synchronization bits. Overhead bits typically appear in a header and trailer to each packet. In addition, acknowledgement packets must be transmitted over the network to confirm receipt of a packet of data. Alternatively, a protocol may include information in the overhead bits in each packet indicating the number of the packet. This information may be used to reassemble the received packets in the correct order, and if a packet is missing, a negative acknowledgement packet may be sent to request retransmission of the missing packet. Otherwise, data loss could occur and not be detected by the system. In any event, acknowledgement packets and other similar handshaking information which must be transmitted over the network according to the protocol impose some limitations upon the data throughput of the network. While this may be acceptable in many instances, in applications where the transfer of huge amounts of data are required, these bandwidth limitations may render such applications impractical in practice.
It is not uncommon for two or more users on a network to attempt to transmit a packet at the same time. When this occurs, it is referred to as a collision. Neither packet will be received successfully, and both must be retransmitted. obviously, this reduces the throughput of the network. Different protocols employ various schemes to determine the timing of retransmission attempts in an effort to avoid repeated collisions between the same two users.
Data transmission may sometimes experience data errors, where a digital "1" is erroneously received as a "0", or vice versa, due to such events as signal fluctuations or noise. Thus, error correction schemes may be employed in an effort to detect data errors. If an error is detected, then a packet must be retransmitted. Of course, when a packet must be retransmitted, it reduces the overall throughput of the network.
Networking technology has suffered from limitations resulting from a proliferation of non-standard protocols, and limitations due to the nature of the protocols and transmission schemes which are employed. Additional overhead may be imposed when conversion from one protocol to another is required. This additional overhead may effectively limit the overall bandwidth of the network.
Networks may need to be connected by hubs, routers, and other switches. A hub, for example, may have a number of ports, and each port may be connected to a network, such as a LAN or a wide area network. When a packet is received at a hub, the hub switch must determine to which port the packet is to be switched. Alternatively, the packet may be switched to all ports and broadcast over every network connected to the hub. However, if every hub broadcasts every packet on every port, the amount of traffic on the network will be increased and the throughput will invariably suffer. Under heavy traffic, any attempt to determine to which port a packet must be switched must be accomplished speedily to avoid slowing throughput of the network. Therefore, it is desirable to have a method for determining over which port a packet should be transmitted.
In addition to limitations on bandwidth, all of the above discussed factors may affect cost, response time, throughput, delay, maximum transmission rates, and reliability.
Some applications, such as full motion video, require transfer of huge amounts of data. Efforts to reduce the performance requirements upon the data transmission system when large amounts of data must be transferred over the system have resulted in various data compression schemes. For example, video or graphical data may be compressed to occupy less space. Compressed data may then be transmitted and, because the data has been compressed into fewer information bits, fewer bits need to be transmitted, thereby relieving to some extent the loading upon the data transmission system. However, there are limits on the extent to which data may be compressed. In the past, compression imposed computational overhead upon the system and sometimes took too much time to complete. Compressed data must be decompressed at the destination at the other end of the transmission system in order to be useable, which imposes additional computational overhead upon the system. Although compression and decompression may be performed in software, the speeds at which such operations can be performed limit the usefulness of such techniques in some applications such as realtime full motion video. If dedicated hardware is utilized, the additional hardware required to perform compression and decompression has limited the use in some applications where small size and miniaturization are required, and in other instances the cost of such additional hardware may be impractical.
Television programming is increasingly being delivered to the consumer by means other than traditional terrestrial broadcast. In the United States, the prime broadcast medium is cable (CATV). At the present time, 90 percent of approximately 93 million TV households in the U.S. are passed by cable--in other words, they could receive cable TV if they chose to subscribe to the cable service. Of these, 55 million, or 60 percent, subscribe to at least a basic cable service. In Europe, the picture is different--roughly 20 percent of all households are passed by cable. Of these, about 60 percent are subscribers. Both Germany and the U.K. have installed bases of 2.5 million direct broadcast by satellite (DBS) satellite dishes. In Asia, DBS services are booming, led by the activities of Star TV in Hong Kong.
The U.S. cable industry consists of two main components: program providers and service operators. The program providers produce the programming (MTV, HBO, Showtime, ESPN), which is distributed by satellite. The local service operator (typically one for each town or city in the U.S.) receives the programming through of satellite dish at what is called a cable headend, and re-transmits it by cable to subscribers. Other functions performed at the headend include receiving and re-transmitting local off-the-air services and local insertion of advertising. These local service operators are typically owned by large corporations known as multiple service operators (MSO). The largest MSOs in the U.S. at present are TCI, Time Warner, Viacom and CableVision. These companies also have interests as Program Providers.
Compressed digital video allows more channels to be transmitted without increasing system bandwidth. Typically 4-10 compressed channels (depending on quality and source) can be transmitted in the space of one conventional channel. This allows for reduction of costs and/or increased capacity.
For the CATV industry, compressed digital video is expected to be rolled out in two phases. The first phase will be the utilization of compressed digital video to deliver programming from the provider to the cable headend. This is motivated by both the cost of satellite transponder rental and by a looming shortage of available slots.
The second phase will be the implementation of compressed digital video to the home. Compressed digital video will give cable operators the ability to deliver as many as 500 channels to the home. The most likely use of these channels will be extended pay per view (PPV) services. The huge channel capacity will allow films to be shown on multiple channels separated by a 10-20 minute interval, thus offering near video on demand to the user.
In its present form, the U.S. CATV industry is close to saturation. Recent regulatory events make it difficult to increase revenues by rate hikes so the industry's best chance for growth is to offer new services, allowing it to compete with the video rental industry and perhaps ultimately with the first run cinema industry. An increase in capacity will allow for additional services which should result in increased revenues.
For direct broadcast by satellite, compressed digital video is the enabling technology which will allow that industry to compete with cable services. Assigning multiple channels to the bandwidth previously required for one both reduces the cost and permits a sufficiently large number of channels to compete with cable services. Two operations planned in the U.S. are Hughes DirecTV and the PRlMESTAR service.
An emerging contender in the market for consumer home video consumption is the telephone companies. The combination of compressed digital video and emerging ADSL (asynchronous digital subscriber line) technology will allow the telephone companies to offer "video dial tone" over twisted pair copper cabling. Combined with a "juke box" of movies, the telephone companies' switched services would allow them to offer true video on demand. Recent regulatory changes have cleared the legal barriers to the telephone companies offering these services.
An emerging market in the U.S. is for subscription-based cable radio, which allows subscribers to receive commercial-free near CD-quality audio-only programming. MPEG audio compression will allow the industry to transmit more channels of higher-quality music through traditional cable lines and use fewer satellite transponders. MPEG refers to the Motion Picture Experts Group which has developed draft standards for audiovisual compression/decompression routines.
Direct audio broadcast (DAB) may be a significant advance in the broadcast industry by transmitting CD-quality music to home and car receivers. Europe and Canada are ahead of the U.S. in the development and implementation of such technology, primarily because of the long approval cycle the FCC requires to approve new transmission bands. Outside of the area of broadcast, several other consumer-oriented applications are emerging that are enabled by compressed digital video, such as compact disc interactive (CD-I) from Philips and similar products from Commodore and Tandy, which are intended to be upgradable to offer full motion video using MPEG compression. CD ROM based technology is also finding its way into video games. The combination of compressed digital video and CD ROM will allow games to feature full motion video.
The combination of compact data streams such as MPEG digital video and audio with networking technology will open up new and useful applications which have not heretofore been practical.